1. Field of the Invention
This invention relates to photographic lenses employing the floating method suited to cameras using photographic material, or video cameras, and more particularly to the extension of the focusing range in such a way as to well correct aberrations for high grade picture quality by using the floating method.
2. Description of the Related Art
In the cameras for photography and video cameras there have been objectives adapted mainly to shoot close objects. These objectives are usually called macro or micro lenses (hereinafter represented by "macro" lenses). The macro lenses are designed to have a high optical performance especially in the closeup range as compared with the other photographic objectives such as standard and telephoto ones. Also, the macro lenses, in many cases, are used even for objects ranging widely from nearby to infinitely distant ones.
In general, when the range of magnification of the macro lens is extended, particularly to the side of higher magnifications, variation of aberrations increases as the magnification varies. It becomes difficult to correct this well.
For example, the macro lens designed in respect to a reproduction ratio of 1/10 when used at a higher value of 1/5 produces very large spherical aberration, curvature of field and coma.
Besides this, when the effective F-number is decreased to make easier focusing, the range of variation of aberrations with variation of the magnification increases in proportion to the lens speed. It becomes difficult to correct this well.
Methods for correcting variation of aberrations with variation of the magnification over a range from an infinitely distant object to a nearby object have been proposed in, for example, Japanese Laid-Open patent applications Nos. SHO 48-90520, 50-138823, 52-7723, 55-28038 and 57-192916 (U.S. Pat. No. 4,553,823). Any of the photographic lenses disclosed in these documents has at least two lens units movable independently of each other when focusing, or utilizes the so-called "floating" technique. However, though each of these lenses is relatively well corrected for aberrations at a nearby object image of low magnification, the correction effect for high magnifications has not always been sufficient. For example, there was a tendency that coma is relatively well corrected at low magnifications, but, as the magnification increases, many aberrations such as distortion and chromatic aberrations are produced.
As another example of application of the floating technique to a closeup range of relatively low magnifications, there is U.S. Pat. No. 3,815,974.